Aromatic carbonate polymers are well known thermoplastic materials which, due to their many advantageous properties, find use as thermoplastic engineering materials. The aromatic polycarbonates exhibit, for example, excellent properties of toughness, impact resistance, heat resistance, and dimensional stability. However, in certain applications the use of aromatic polycarbonate resins is limited because they exhibit environmental stress crazing and cracking. "Environmental stress crazing and cracking" refers to the type of failure which is hastened by the presence of organic solvents such as, for example, gasoline, particularly high octane no lead gasoline, acetone, heptane and carbon tetrachloride when such solvents are in contact with the stressed parts fabricated from aromatic polycarbonates. The most significant effect is loss of vital impact strength and also an increase in brittle-type failure. Contact with such solvents may occur, for example, when parts are used under the hood of automobiles or near the gasoline filler ports thereof, or when solvents are used to clean or degrease stressed parts made from polycarbonate resins.
In Goldblum, U.S. Pat. No. 3,431,224, assigned to the same assignee as the instant application, it is proposed to add modifiers to polycarbonates, in certain proportions, the modifiers comprising at least one member selected from the class consisting of polyethylene, polypropylene, polyisobutylene, a copolymer of ethylene and an ethyl acrylate, a copolymer of ethylene and propylene, a cellulose ester, a polyamide, a polyvinyl acetal, an alkyl cellulose ether, and a polyurethane elastomer. While the results with such modifiers are generally excellent, in thin section, e.g., 1/8 inch, it has been found that there is a tendency for failure to occur with these modifiers in thicker molded parts, e.g., 1/4 inch thickness, and such failure is of the undesirable brittle type, especially after exposure to high test gasoline. Another modifier proposed to be added to polycarbonate is reported in Research Disclosure No. 20810, Dow Chemical Company, August, 1981. Data are provided showing that polycarbonate modified with a linear low density polyolefin, namely ethylene/octene-1 copolymer, provide good impact strength at increased part thickness. There is no suggestion therein that such a modifier will enhance resistance to environmental stress crazing and cracking. Still other modifiers have been proposed for impact strength improvement, and for slight improvement in optimum environmental stress crazing and cracking resistance. U.S. Pat. Nos. 4,444,949, 4,430,476, and 4,481,331. U.S. Pat. No. 4,444,949 describes polycarbonates modified with a combination of butadiene-styrene coupled block copolymer and, optionally, an olefin/acrylate copolymer. Such compositions process well and are toughened, but there is no disclosure of significant solvent resistance. U.S. Pat. No. 4,430,476 describes polycarbonates modified with a combination of the coupled block copolymers and a linear low density polyolefin. There is no mention that such modifier combinations will provide enhanced resistance to crazing and cracking. U.S. Pat. No. 4,481,331 describes polycarbonate modified with a vinyl aromatic and olefin elastomeric block copolymer and an olefin/acrylate copolymer or with a vinyl aromatic and olefin elastomeric block copolymer, an olefin/acrylate copolymer, and a polyolefin resin. Such compositions are toughned and exhibit an improvement in stress crazing and cracking.